Vlasak came to public attention in 2003, and 2004 and again in 2008, when he made statements that appeared to justify the use of violence against animal researchers.[3] He responded that he had been quoted out of context and strongly denied having said anything to encourage violent action. Nevertheless, as a result of his statements, he and his wife were banned from entering the United Kingdom in 2004, on the grounds that their presence, according to the Home Secretary, "would not be conducive to the public good."[2]

He was an animal researcher himself, and saw no problem with it at the time. According to the Los Angeles Times, he conducted research on dogs' arteries in a laboratory at the Harbor-UCLA Medical Center, during or after which the dogs were killed.[5]

His principal role in the animal liberation movement is as a liaison between the movement and the public, publicizing the movement's "underground" activities in his role as a press officer. He acknowledges his medical background provides a "certain amount of credibility" to the movement.[8]

Vlasak explained his opposition to animal testing to a U.S. Senate committee in 2005:

“

Here in the U.S., there are thousands of physicians like myself who realize there is no need to kill animals in order to help humans, the vast majority of whom get sick and die because of preventable lifestyle variables such as diet, smoking, drugs and environmental toxins. In a country where 45 million people do without reliable access to ANY medical care, there is no reason to waste hundreds of millions of dollars testing drugs and procedures on non-human animals. In a world where 20,000 children are dying from lack of access to clean water each week world wide, there is no reason to waste hundreds of millions of dollars testing drugs and procedures on non-human animals.[9]

Vlasak has been active in opposing the seal hunt in Canada, which occurs every year, mostly in March and April. He was punched in the face by sealers during the 2005 hunt on Prince Edward Island, which he attended on behalf of the Sea Shepherd Conversation Society. No charges were brought in connection with the attack.[10] Vlasak was Sea Shepherd's treasurer for many years.

During the same hunt, he was one of 11 activists convicted of violating Canada's Department of Fisheries and Oceans regulations, because they watched the hunt without a permit. He was sentenced to 22 days in prison, which he served in a Prince Edward Island jail in 2006.[11]

Vlasak was convicted in Los Angeles in 2006 of "targeted protesting" for demonstrating against euthanasia at animal shelters outside the home of a Department of Animal Services employee. He was sentenced to 30 days' electronic monitoring. He has appealed the decision.[12]

Vlasak has been criticized for appearing to legitimize the use of violence against animal researchers. He told an animal rights conference in 2003:

I think there is a use for violence in our movement. And I think it can be an effective strategy. Not only is it morally acceptable, I think that there are places where it could be used quite effectively from a pragmatic standpoint.

For instance, if vivisectors were routinely being killed, I think it would give other vivisectors pause in what they were doing in their work — and if these vivisectors were being targeted for assassination ... — and I wouldn't pick some guy way down the totem pole, but if there were prominent vivisectors being assassinated, I think that there would be a trickle-down effect and many, many people who are lower on that totem pole would say, "I'm not going to get into this business because it's a very dangerous business ...

And I don't think you'd have to kill — assassinate — too many vivisectors before you would see a marked decrease in the amount of vivisection going on. And I think for 5 lives, 10 lives, 15 human lives, we could save a million, 2 million, 10 million non-human animals.

And I — you know — people get all excited about, "Oh what's going to happen when the ALF accidentally kills somebody in an arson?" Well, you know I mean, I think we need to get used to this idea. It's going to happen, okay? It's going to happen.[13]

Following the August 2, 2008 firebombing of a house belonging to University of California Santa Cruz animal researchers, Vlasak was quoted as saying "This guy knows what he is doing. He knows that every day that he goes into the laboratory and hurts animals that it is unreasonable not to expect consequences." [14]

The controversy continued in July 2004 when Jamie Doward of The Observer wrote that Vlasak had told him during an interview: "I think violence is part of the struggle against oppression. If something bad happens to these people [animal researchers], it will discourage others. It is inevitable that violence will be used in the struggle and that it will be effective."[15]

Vlasak responded in a press release that the allegations were part of a smear campaign against him. He wrote: "I was outraged by Jamie Doward's article in the Observer stating that I, a medical doctor who spends my entire life SAVING lives, is calling for or encouraging the assassination or killing of any being, human or non human." He continued:

People have been killed over absolutely ridiculous things like oil, power and money. It would be "speciesist" of me to say that in a battle for the moral and ethical high ground ... that there will never be casualties. I'm not encouraging or calling for this, I am simply stating that the animal rights movement is and has been the most peaceful and restrained movement the world has ever known considering the amount of terror, abuse and murder done to innocent animals for greed and profit. If by chance violence is used by those who fight for non human sentient beings, or even if there are casualties, it must be looked at in perspective and in a historical context.[16]

He told the BBC's Today program: "I am personally not advocating, condoning or recommending that anybody be killed. I am a physician who saves lives. I spend my entire day saving people's lives. All I am saying, in a historical context, violence has been used against us as animal rights campaigners and against the animals and is no different from us using violence on the other side."[17]

On Australian television a few months later he said: "Would I advocate taking five guilty vivisectors' lives to save hundreds of millions of innocent animal lives? Yes, I would."[18]

As a result of the Observer article, the British government announced in August 2004 that Vlasak and his wife were banned from the UK. Vlasak had been due to attend a conference organized by Stop Huntingdon Animal Cruelty (SHAC), an animal-rights campaign to close Huntingdon Life Sciences. The Home Secretary told the couple that their presence in the UK "would not be conducive to the public good."[2]

Vlasak is one of two animal liberation press officers who have been banned from entering the UK. Steven Best, professor of philosophy at the University of Texas, El Paso, was banned in 2005 to prevent him from addressing an animal rights conference there.[19]